Theory vs. theory. Is a scientific
theory
different from the everyday use of the word
theory?

The Big Bang Theory. The Theory of Gravity. The Plate Tectonic Theory. The Modern Atomic Theory. The Cell Theory. The Theory of Evolution by Natural Selection. These are all classic scientific theories. So, without a doubt, yes, a scientific
theory
is very different from the everyday use of the word
theory.
A scientific theory is accepted as a scientific
truth,
supported by evidence collected by many scientists. Lots of data has been collected to support theories, and no data has been identified to prove theories incorrect. That does not necessarily mean that evidence does not exist against a theory does not exist, it simply means that evidence has yet to be identified.

Scientific Theories

Scientific theories
are hypotheses which have stood up to repeated attempts at falsification and are thus supported by a great deal of data and evidence. Some well known biological theories include the theory of evolution by natural selection, the cell theory (the idea that all organisms are made of cells), and the germ theory of disease (the idea that certain microbes cause certain diseases). The scientific community holds that a greater amount of evidence supports these ideas than contradicts them, and so they are referred to as theories. In fact, no evidence has been identified to disprove these or other scientific theories.

In every day use, people often use the word
theory
to describe a guess or an opinion. For example, “I have a theory as to why the light bulb is not working.” When used in this common way, “theory” does not have to be based on facts. It does not have to be based on a true description of reality. This usage of the word theory often leads to a misconception that can be best summed up by the phrase, "It's not a fact, it's only a theory." In such everyday usage, the word is most similar to the term hypothesis.

Scientific theories are the equivalent of what in everyday speech we would refer to as
facts
. In principle, scientific theories are always subject to corrections or inclusion in another, wider theory. As a general rule for use of the term, theories tend to deal with broader sets of phenomena than do hypotheses, which usually deal with much more specific sets of phenomena or specific applications of a theory.

Constructing Theories

In time, a confirmed hypothesis may become part of a theory or may grow to become a theory itself. Scientific hypotheses may be mathematical models. Sometimes they can be statements, stating that some particular instance of the phenomenon under examination has some characteristic and causal explanations. These theories have the general form of universal statements, stating that every instance of the phenomenon has a particular characteristic.

A hypothesis may predict the outcome of an experiment in a laboratory or the observation of a natural phenomenon. A hypothesis should also be falsifiable, and one cannot regard a hypothesis or a theory as scientific if it does not lend itself to being falsified, even in the future. To meet the “falsifiable” requirement, it must at least in principle be possible to make an observation that would disprove the hypothesis. A falsifiable hypothesis can greatly simplify the process of testing to determine whether the hypothesis can be proven to be false. Scientific methods rely heavily on the falsifiability of hypotheses by experimentation and observation in order to answer questions. Philosopher Karl Popper suggested that all scientific theories should be falsifiable, or otherwise they could not be tested by experiment.

A
scientific theory
must meet the following requirements:

it must be consistent with a pre-existing theory. The pre-existing theory must have been experimentally verified, though it may often show a pre-existing theory to be wrong in an exact sense,

it must be supported by many strands of evidence rather than a single foundation, ensuring that it is a good approximation or even completely correct.

Also, a theory is generally only taken seriously if it:

allows for changes to be made as new data is discovered, rather than claiming absolute certainty,

is the most straight forward explanation, and makes the fewest assumptions about a phenomenon (commonly called “passing the Occam's razor test”).

This is true of such established theories as those of special relativity, general relativity, quantum mechanics, plate tectonics, and evolution. Theories considered scientific ideally meet all of these extra criteria as well.

In summary, to meet the status of a scientific theory, the theory must be falsifiable or testable. Examples of scientific theories in different areas of science include:

Physics
: General Relativity; Special Relativity; Theory of Relativity; Quantum Field Theory

Earth Science
: Giant Impact Theory; Plate Tectonics

Unverifiable Theories

The term theory is sometimes stretched to refer to theoretical speculation which is currently unverifiable. A couple examples of such uses include string theory and the theory of everything. String theory is a model of physics, which predicts the existence of many more dimensions in the universe than the four dimensions that current science understands (length, width, height, and space-time). The theory of everything is a hypothetical theory in physics that fully explains and links together all known physical phenomena.

For a scientific theory to be valid, it must be verified experimentally. Many parts of the string theory are currently untestable due to the large amount of energy that would be needed to carry out the necessary experiments as well as the high cost of conducting these experiments. Therefore string theory may not be tested in the foreseeable future. Some scientists have even questioned whether it deserves to be called a scientific theory because it is not falsifiable.

Superseded Theories

A
superseded theory
, or obsolete scientific theory is a theory that was once commonly accepted, but for a given reason is no longer considered the most complete description of reality by mainstream science. It can also refer to a falsifiable theory which has been shown to be false. Giraffes, shown in
Figure
below
, are often used in the explanation of Lamarck's superseded theory of evolution. In Lamarckism, a giraffe lengthens its neck over the course of its life in order to, for example, reach higher leaves. That giraffe will then have offspring with longer necks. The theory has been superseded by the understanding of natural selection on populations of organisms as the main means of evolution (Darwin's theory of evolution by natural selection), not physical changes to a single organism over its lifetime.

Superseded theories like Lamarck’s theory of evolution are theories that are now considered obsolete and have been replaced by newer theories that have more evidence to support them. In Lamarck’s case, his theory was replaced by Darwin’s theory of evolution and natural selection, which will be further discussed in additional concepts.

Scientific Laws

Scientific laws
are similar to scientific theories, in that they are principles which can be used to predict the behavior of the natural world. Both scientific laws and scientific theories are typically well-supported by observations and/or experimental evidence. Usually scientific laws provide rules for how nature will behave under certain conditions. Scientific theories are more overarching explanations of how nature works and why it exhibits certain characteristics.

A
physical law
or
law of nature
is a scientific generalization based on a sufficiently large number of empirical observations, so that it is accepted as fully verified.

Isaac Newton's law of gravitation is a famous example of an established law that was later found not to be universal—it does not hold in experiments involving motion at speeds close to the speed of light or in close proximity of strong gravitational fields. However, outside these conditions, Newton's laws remain an excellent model of motion and gravity.

Scientists never claim absolute knowledge of nature or the behavior of the subject of the field of study. A scientific theory is always open to falsification, if new evidence is presented. Even the most basic and fundamental theories may turn out to be imperfect if new observations are inconsistent with them. It is critical to make every relevant part of research publicly available. This allows for and encourages peer review of published results, and it also allows ongoing reviews, repetition of experiments and observations by many different researchers. Only by meeting these expectations can it be determined how reliable the experimental results are for possible use by others.

Vocabulary

law of nature
: A scientific generalization based on a sufficiently large number of empirical observations so that it is taken as fully verified; physical law.

physical law
: A scientific generalization based on a sufficiently large number of empirical observations so that it is taken as fully verified; law of nature.

scientific law
: A principle which can be used to predict the behavior of the natural world.

scientific theory
: A hypothesis which has stood up to repeated attempts at falsification; supported by a great deal of data and evidence.

superseded theory
: A theory that was once commonly accepted, but is no longer considered the most complete description of reality by mainstream science.

Summary

Scientific theories are hypotheses which have stood up to repeated attempts at falsification and are thus supported by much data and evidence.

Scientific laws are similar to scientific theories in that they are principles which can be used to predict the behavior of the natural world.

A scientific theory must be supported by many strands of evidence rather than a single foundation, ensuring that it is probably a good approximation, if not totally correct.

A superseded scientific theory is a theory that was once commonly accepted, but is no longer considered the most complete description of reality by mainstream science.